1. Field of the Invention
The invention relates to a fuel cell having a stack structure.
2. Description of the Related Art
A fuel cell is proposed, which includes a hydrogen electrode and an oxygen electrode provided on both sides of an electrolyte membrane which a hydrogen ion permeates, and a reaction represented by an equation described below is caused at each of the hydrogen electrode and the oxygen electrode, whereby electromotive force is generated.
Hydrogen electrode (anode): H2→2H++2e− (anode reaction)
Oxygen electrode (cathode): (1/2) O2+2H++2e−→H2O (cathode reaction)
Various types of fuel cells are proposed according to the type of electrolyte membrane. For example, a solid oxide fuel cell, a molten carbonate fuel cell, a phosphoric acid fuel cell, and a polymer electrolyte fuel cell are proposed. Recently, attention has been given to the polymer electrolyte fuel cell, for the reasons that electric power density is high so that the size can be made small, and the operating temperature is relatively low, and other reasons. Various improvements of the polymer electrolyte fuel cell are examined.
There is a fuel cell having a stack structure, which is formed by stacking plural unit cells through separators. Each of the unit cells has a gas diffusible anode and a gas diffusible cathode which are provided on both sides of an electrolyte membrane. In the fuel cell having the stack structure (hereinafter, referred to as “fuel cell stack”), the temperature of unit cells positioned at both end portions in a direction in which the unit cells are stacked (hereinafter, referred to as “cell-stacked direction”) tends to be lower than that of unit cells positioned at a center portion due to heat radiation. Therefore, in the unit cells at both end portions of the fuel cell stack, vapor pressure is reduced, and water generated by the aforementioned reaction is likely to be accumulated.
In the polymer electrolyte fuel cell, it is important to control water in each of the unit cells in order to ensure conductivity of the hydrogen ion in the electrolyte membrane, and to ensure gas diffusibility at a gas diffusion electrode formed by stacking a catalyst layer and a gas diffusion layer. For example, if the amount of water contained in the electrolyte membrane decreases, the conductivity of the hydrogen ion decreases, and accordingly electric power generation performance of the fuel cell decreases. Also, if generated water is accumulated at the gas diffusion electrode, so-called flooding occurs, and the gas diffusibility decreases. As a result, the electric power generation performance of the fuel cell decreases. Thus, various technologies concerning control of water in the unit cells of the polymer electrolyte fuel cell are proposed.
For example, Japanese Patent Laid-Open Publication No. 9-92322 discloses a related technology in which a flow amount of oxidizing agent gas is increased at both end portions of a fuel cell stack, and the flow amount of oxidizing agent gas is decreased at a center portion of the fuel cell stack, whereby amounts of water contained in plural electrolyte membranes are made uniform.
Also, Japanese Patent Laid-Open Publication No. 2001-357869 discloses a technology in which water repellency of cathode gas diffusion layers of unit cells positioned at both end portions of a fuel cell stack is made low as compared with unit cells positioned at other portions, or gas permeability of the cathode gas diffusion layers of the unit cells positioned at both end portions is made high as compared with the unit cells positioned at other portions, whereby accumulation of excessive water in cathode catalyst layers is suppressed.
In all of these technologies, a decrease in the electric power generation performance of the fuel cell is suppressed by controlling the wet state of the unit cells.
However, in the technology disclosed in the aforementioned Japanese Patent Laid-Open Publication No. 9-92322, there is a problem that the control of the flow amount of oxidizing agent gas becomes complicated. Also, in the technology disclosed in the aforementioned Japanese Patent Laid-Open Publication No. 2001-357869, a process of producing the gas diffusion layer becomes complicated considering the water repellency and gas permeability.